1. Field of the Invention
The present invention relates to an optical modulator having intensity balance function, and the like. In particular, the present invention relates to an optical modulator and the like which is capable of adjusting optical intensity of optical signals which contains non-desired components so that the intensity of the components become at a similar level, whereby the optical modulator is capable of effectively suppressing the non-desired components when the optical signals are combined.
2. Description of the Related Art
In optical communication, light must be modulated to have signals. As optical modulation, direct modulation and external modulation are known. The direct modulation is one modulating a driving power of semiconductor laser. And the external modulation is one modulating light from semiconductor laser by means other than light source. A modulator used in direct modulation is generally called an optical modulator. The optical modulator modulates optical intensity, phase, etc. by causing physical changes in the optical modulator based on signals. As technical problems of the optical modulator, there exist reduction of driving voltage, realization of a higher extinction ratio for improving modulation efficiency, widening a bandwidth, and improvement of high light utilization efficiency for speeding up and loss reduction of a modulation. In other words, development of a modulator having high extinction ratio is desired. It is to be noted that the extinction ratio is a ratio of optical intensity between the highest level to the lowest level.
As a modulator which shifts frequency of an optical signal to output the optical signal, there is an optical signal side-band (optical SSB) modulator [Tetsuya Kawanishi and Masayuki Izutsu, “Optical frequency shifter using optical SSB modulator”, TECHNICAL REPORT OF IEICE, OCS2002-49, PS2002-33, OFT2002-30 (2002-08)].
An optical FSK modulator which is a modification of an optical SSB modulator is also known [Tetsuya Kawanishi and Masayuki Izutsu, “Optical FSK modulator using an integrated light wave circuit consisting of four optical phase modulator”, CPT 2004G-2, Tokyo, Japan, 14-16 Jan. 2004] [Tetsuya Kawanishi, et al. “Analysis and application of FSK/IM simultaneous modulation” Tech. Rep. of IEICE. EMD 2004-47, CPM 2004-73, OPE 2004-130, LQE 2004-45 (2004-08), pp. 41-46].
FIG. 9 is a schematic diagram showing a basic arrangement of a conventional optical modulation system acting as an optical SSB modulator or an optical FSK modulator. As shown in FIG. 9, this optical modulation system comprises a first sub Mach-Zehnder waveguide (MZA) (2), a second sub Mach-Zehnder waveguide (MZB) (3), a main Mach-Zehnder waveguide (MZC) (8), a first electrode (RFA electrode) (9), a second electrode (RFB electrode) (10), and a modulation electrode.
The main Mach-Zehnder waveguide (MZC) (8) includes an input part (4) of an optical signal, a branching part (5) where the optical signal is branched to the first sub Mach-Zehnder waveguide (MZA) and the second sub Mach-Zehnder waveguide (MZB), the first sub Mach-Zehnder waveguide (MZA), the second sub Mach-Zehnder waveguide (MZB), a combining part (6) combining the optical signals outputted from the first sub Mach-Zehnder waveguide (MZA) and the second sub Mach-Zehnder waveguide (MZB), an output part (7) outpuffing the optical signal combined at the combining part.
The first electrode (RFA electrode) (9) inputs radio frequency (RF) signals to two arms composing the first sub Mach-Zehnder waveguide (MZA).
The second electrode (RFB electrode) (10) inputs radio frequency (RF) signals to two arms composing the second sub Mach-Zehnder waveguide (MZB).
The modulation electrode is provided on the main Mach-Zehnder waveguide.
Changing USB and LSB, which can be used as information, are attained by means of electrode of the main Mach-Zehnder waveguide; thereby frequency shift keying is realized.
As an optical modulator, an optical double side-band suppressed carrier (DSB-SC) modulator is publicly known. The above described optical modulation system also acts as a DSB-SC modulator. The DSB-SC modulator ideally outputs two side bands, suppressing carrier components. However, in reality, in an output of a DSB-SC modulator shown in the figure below, unsuppressed carrier component (f0) and a high order component (e.g. a second order component (f0±2fm)) remain, preventing extinction ratio from improving.
As a DSB-SC modulator, for example, a DSB-SC modulator having a Mach-Zehnder, PMs provided on both arms of the Mach-Zehnder and a fixed phase shifter provided on one arm of the Mach-Zehnder is disclosed in FIG. 37 of Japanese Unexamined Patent Application Publication No. 2004-252386. FIG. 10 shows an optical modulator described in FIG. 37 of Japanese Unexamined Patent Application Publication No. 2004-252386. An optical DSB-SC modulator ideally outputs two sideband (double sideband) signals, thereby suppressing carrier signal components. However, in the actual output of an optical DSB-SC modulator, other than side band signals, there remain unsuppressed carrier components and high order component signals, preventing extinction ratio from improving. Therefore, traditional optical DSB-SC modulator was aimed to output an optical signal with suppressed carrier component and suppressed high order components.
One of the reasons that there remain a carrier component (f0) and a high order component (e.g. a second order component (f0±2fm) which cannot be suppressed completely is considered to be as follows. Outputs from each sub Mach-Zehnder waveguide are combined, but intensity of a carrier component (f0) and a high order component (e.g. a second order component (f0±2fm) of an output signal from one sub Mach-Zehnder waveguide is not always equal to intensity of corresponding components of an output signal from another corresponding sub Mach-Zehnder waveguide. Therefore, the components remain without being suppressed sufficiently when the outputs are combined.
It is an object of the present invention to provide an optical modulator which is capable of adjusting optical intensity of optical signals which contains non-desired components so that the intensity of the components become at a similar level, whereby the optical modulator is capable of effectively suppressing the non-desired components when the optical signals are combined.